1. Field of the Invention
This invention relates to a DC/DC converter which can produce a plurality of stable output voltages; and more particularly, to such a DC/DC converter wherein a DC voltage supplied to a primary coil is selectively turned ON and OFF to cause the output voltages to be at a constant level.
2. Description of the Prior Art
Power supplies are often required to produce a plurality of output voltages, such as 5 V, 12 V and 28 V. To reduce cost, such power supplies usually comprise multi-output converters.
FIG. 1 depicts a conventional stabilized power supply incorporating a multi-output ON/ON type converter. The converter comprises transformer T1 having a primary coil to which input DC voltage Vin is applied and secondary coils n.sub.2, n.sub.3. Diodes D.sub.1,D.sub.2 connected to coil n.sub.2 rectifies AC voltage produced in secondary coil n.sub.2. Diodes D.sub.3,D.sub.4 connected to coil n.sub.3 rectifies AC voltage produced in secondary coil n.sub.3. Filters, or smoothing circuits, comprising smoothing coils L.sub.1,L.sub.2 and smoothing capacitors C.sub.1, C.sub.2 are connected to the rectifiers as depicted. The outputs from these smoothing circuits comprise the power outputs V.sub.o1 and V.sub.o2.
A comparator U.sub.1 having inputs from the upper output terminal and source of voltage V.sub.r, as depicted, is adapted to compare V.sub.o1 with reference voltage Vr. Comparator U.sub.1 is connected to pulse width modulating (PWM) circuit 1 which is connected to the comparator U.sub.1 as depicted, and is adapted to produce upon receipt of the output from comparator U.sub.1, a pulse width signal corresponding to the difference between output V.sub.o1 and reference voltage V.sub.r. The PWM circuit 1 is connected to drive circuit 2 which is adapted to receive the ouput from PWM circuit 1. A switching transistor Q.sub.1 is connected to drive circuit 2 at its base, and to input source and primary n.sub.1, at its emitter and collector. Transistor Q.sub.1 is turned ON and OFF by drive circuit 2 so as to turn ON and OFF input DC voltage Vin supplied to the primary coil n.sub.1 of transformer T.sub.1.
In this prior apparatus, one voltage output V.sub.o1, of the two outputs V.sub.o1 and V.sub.o2, is compared with the reference voltage V.sub.r by comparator U.sub.1, and transistor Q.sub.1 is turned ON and OFF through action of PWM circuit 1 and driving circuit 2, so that the output voltage V.sub.o1 takes a constant level, corresponding to the level of the reference voltage V.sub.r, thereby to stabilize the output voltage V.sub.o1 and substantially stabilize output voltage V.sub.o2.
This prior apparatus, however, suffers from a disadvantage in that the output voltage V.sub.o2 fluctuates over a wide range depending on the combination of loads connected to the two output terminals (labelled for convenience only as V.sub.o1 and V.sub.o2). Thus, it is known that prior devices leave much to be improved upon.